U.S. patent number 9,991,959 [Application Number 15/296,927] was granted by the patent office on 2018-06-05 for frequency comparison and phase synchronization in optical signals.
This patent grant is currently assigned to NATIONAL INSTITUTES OF NATURAL SCIENCES. The grantee listed for this patent is NATIONAL INSTITUTES OF NATURAL SCIENCES. Invention is credited to Hitoshi Kiuchi.
United States Patent |
9,991,959 |
Kiuchi |
June 5, 2018 |
Frequency comparison and phase synchronization in optical
signals
Abstract
An optical oscillator 10 combines optical signals L1' and L2 to
generate an optical signal L3. The optical signal L1' includes
light waves W1' and W2' having frequencies spaced apart by a
frequency difference .DELTA.f1. The optical signal L2 includes
light waves W3 and W4 having frequencies spaced apart by a
frequency difference .DELTA.f2. The optical oscillator 10 separates
the optical signal L3 into optical signals L4 and L5, wherein the
optical signal L4 includes the light waves W1' and W3 and the
optical signal L5 includes the light waves W2' and W4. The optical
oscillator 10 compares the frequency differences .DELTA.f1 and
.DELTA.f2 based on frequency difference .DELTA.f3 between the light
waves W1' and W3 included in the optical signal L4 and frequency
difference .DELTA.f4 between the light waves W2' and W4 included in
the optical signal L5.
Inventors: |
Kiuchi; Hitoshi (Tokyo,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NATIONAL INSTITUTES OF NATURAL SCIENCES |
Tokyo |
N/A |
JP |
|
|
Assignee: |
NATIONAL INSTITUTES OF NATURAL
SCIENCES (Tokyo, JP)
|
Family
ID: |
58714693 |
Appl.
No.: |
15/296,927 |
Filed: |
October 18, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170214464 A1 |
Jul 27, 2017 |
|
Foreign Application Priority Data
|
|
|
|
|
Jan 21, 2016 [JP] |
|
|
2016-009701 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B
10/691 (20130101); H04L 7/0075 (20130101); H04B
10/25891 (20200501); H04J 3/00 (20130101); H01S
3/1304 (20130101); H01S 3/1307 (20130101); H01S
3/1305 (20130101) |
Current International
Class: |
H04B
10/25 (20130101); H04B 10/69 (20130101); H04L
7/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Tran; Dzung
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Claims
What is claimed is:
1. A method for comparing frequency differences between optical
signals, comprising: combining a first optical signal and a second
optical signal to generate a third optical signal, wherein the
first optical signal includes a first light wave and a second light
wave having frequencies spaced apart by a first frequency
difference and wherein the second optical signal includes a third
light wave and a fourth light wave having frequencies spaced apart
by a second frequency difference; separating the third optical
signal into a fourth optical signal and a fifth optical signal,
wherein the fourth optical signal includes the first and third
light waves and the fifth optical signal includes the second and
fourth light waves; and comparing the first frequency difference
and the second frequency difference based on a third frequency
difference and a fourth frequency difference, wherein the third
frequency difference is a frequency difference between the first
and third light waves included in the fourth optical signal and the
fourth frequency difference is a frequency difference between the
second and fourth light waves included in the fifth optical signal
by generating a first electric signal having a frequency
corresponding to the third frequency difference based on the fourth
optical signal, generating a second electric signal having a
frequency corresponding to the fourth frequency difference based on
the fifth optical signal, and detecting a phase difference between
the first and second electric signals, wherein the first frequency
difference is variable and controlled, the second frequency
difference is a reference as a goal of control; and changing the
first frequency difference based on the phase difference.
2. The method of claim 1, further comprising shifting the frequency
of the first or second optical signal before generating the third
optical signal.
3. The method of claim 1, wherein changing the first frequency
difference comprises: generating an optical comb having a frequency
interval corresponding to the phase difference; and extracting the
first and second light waves from the optical comb, and wherein the
method further comprises extracting two light waves, from the
optical comb, including at least one light wave which is different
from either of the first and second light waves.
4. A device for synchronizing phases of optical signals comprising:
a two light wave generator for generating the first optical signal,
wherein the first optical signal includes a first light wave and a
second light wave having frequencies spaced apart by a first
frequency difference that is variable and controlled; an optical
coupler for combining the first and second optical signals, the
optical coupler combining a first optical signal and a second
optical signal to generate a third optical signal, and wherein the
second optical signal includes a third light wave and a fourth
light wave having frequencies spaced apart by a second frequency
difference; and an optical separator for separating the third
optical signal into the fourth and fifth optical signals by
separating the third optical signal into a fourth optical signal
and a fifth optical signal, wherein the fourth optical signal
includes the first and third light waves and the fifth optical
signal includes the second and fourth light waves; and an optical
comparator for comparing the first frequency difference and the
second frequency difference based on a third frequency difference
and a fourth frequency difference, wherein the third frequency
difference is a frequency difference between the first and third
light waves included in the fourth optical signal and the fourth
frequency difference is a frequency difference between the second
and fourth light waves included in the fifth optical signal by
generating a first electric signal having a frequency corresponding
to the third frequency difference based on the fourth optical
signal, generating a second electric signal having a frequency
corresponding to the fourth frequency difference based on the fifth
optical signal, and detecting a phase difference between the first
and second electric signals, wherein the first frequency difference
is variable and controlled, the second frequency difference is a
reference as a goal of control; and changing the first frequency
difference based on the phase difference.
5. The device of claim 4, further comprising: a first photodetector
for generating a first electric signal having a frequency
corresponding to the third frequency difference based on the fourth
optical signal; and a second photodetector for generating a second
electric signal having a frequency corresponding to the fourth
frequency difference based on the fifth optical signal, and wherein
upper limits of detectable frequency ranges for the first and
second photodetectors are lower than the first frequency
difference.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention is related to a method for comparing
frequency differences between optical signals, a method for
synchronizing phases of optical signals and a device for
synchronizing phases of optical signals.
Description of the Related Art
It is possible to transmit highly stable high-frequency reference
signals for long distances by using two coherent laser waves. This
allows construction of extensive coherent systems. A high-frequency
reference signal, transmitted as a beat signal between two light
waves, is subjected to optical-electric signal conversion, and then
phase-synchronized with an electric oscillator by using a phase
synchronization circuit so that a new frequency is generated.
Today, reference signals transmitted by two light waves span a wide
frequency band ranging from low frequencies to a frequency of more
than 100 GHz. In order to do this, various types of photomixers or
frequency converters used for optical-electric signal conversion
have to be provided ranging from those for low frequencies to those
for high frequencies. A good example of this is ALMA (Atacama Large
Millimeter/submillimeter Array). The construction of ALMA is
described in J. F. Cliche, et al, "A 100-GHz-tunable photonic
millimeter wave synthesizer for the Atacama Large Millimeter Array
radio telescope", IEEE/MTT-S International Microwave Symposium,
2007, p. 349-352.
Great developments are expected in the future in the field of
generating highly phase-stable signals from the microwave region to
the terahertz wave region. The need thereof is growing in high
speed optical communication, high-frequency astronomy, etc.
Using photonic technologies is advantageous for generating
high-frequency wideband signals. Also, optical signals can be
easily distributed for long distances by using fiber transmission.
Recently, systems constituting an extensive coherent system by
distributing a high-frequency optical reference signal in a broad
area are beginning to be realized. A reference signal in a high
frequency is advantageous for maintaining coherence because the
signal does not require any multiplier or the like at the
distributed destination. A good example of this is the ALMA. A
high-frequency optical reference signal is generated and
transmitted as a beat signal between two coherent laser waves. In
conventional techniques, this high-frequency optical reference
signal is converted into an electric signal by optical-electric
signal conversion using a photomixer or the like to provide a
frequency reference, which is used for comparison with a controlled
signal as to their frequency differences.
FIG. 5 shows an example of a conventional frequency comparison
method. This construction corresponds to a scheme referred to as
PLL (Phase Locked Loop). A "two light wave generation means"
generates an optical signal including two light waves. The optical
signal is distributed in two paths by an "optical distributor". One
of the distributed optical signals is subjected to optical-electric
conversion and an output signal is outputted as a high-frequency
electric signal having a frequency corresponding to a frequency
difference between the two light waves. The other signal is also
subjected to optical-electric conversion and a phase difference
between this and a reference optical signal, which has been
subjected to optical-electric conversion in a similar manner, is
detected. The frequency difference of the two light waves (the
optical signal) generated by the "two light wave generation means"
is controlled based on this phase difference.
FIG. 6 shows another example of a conventional frequency comparison
method. This construction represents a PLL oscillator synchronized
with an optical reference signal. This example uses a
"high-frequency electric signal generation means" instead of the
"two light wave generation means". The signal from the
"high-frequency electric signal generation means" is distributed in
two paths, one of which is an output signal. A phase difference
between the other path and an optical reference signal, subjected
to optical-electric conversion, is detected. The frequency of the
electric signal generated by the "high-frequency electric signal
generation means" is controlled based on this phase difference.
Thus, in both the conventional examples, an optical-electric
converter is required for detecting the frequency difference
between two light waves at least in the optical reference signal.
This optical-electric converter has to be operational over an
entire range of possible frequencies for the optical reference
signal.
SUMMARY OF THE INVENTION
However, conventional methods such as the above have a problem in
that the number of converters required for performing
optical-electric signal conversion increases in response to the
breadth of the frequency range for the reference signal transmitted
by the two light waves.
Today, reference signals transmitted by two light waves span a
broad frequency band ranging from low frequencies to frequencies of
more than 100 GHz. Accordingly, a broad band system has to provide
various types of converters for performing optical-electric signal
conversion (high-frequency photomixer, high-frequency mixer for
frequency conversion (such as a harmonic mixer), etc.) from those
for low frequencies to those for high frequencies. As a result, the
construction of the system becomes complicated and may also become
expensive.
The present invention is made in order to solve these problems and
is aimed at providing a method for comparing frequency differences
that reduces the number of converters required for performing
optical-electric conversion.
In order to solve these problems, a method for comparing frequency
differences between optical signals related to the present
invention comprises: combining a first optical signal and a second
optical signal to generate a third optical signal, wherein the
first optical signal includes a first light wave and a second light
wave having frequencies spaced apart by a first frequency
difference and wherein the second optical signal includes a third
light wave and a fourth light wave having frequencies spaced apart
by a second frequency difference; separating the third optical
signal into a fourth optical signal and a fifth optical signal,
wherein the fourth optical signal includes the first and third
light waves and the fifth optical signal includes the second and
fourth light waves; and comparing the first frequency difference
and the second frequency difference based on a third frequency
difference and a fourth frequency difference, wherein the third
frequency difference is a frequency difference between the first
and third light waves included in the fourth optical signal and the
fourth frequency difference is a frequency difference between the
second and fourth light waves included in the fifth optical
signal.
The method may further comprise shifting the frequency of the first
or second optical signal before generating the third optical
signal.
Comparing the first frequency difference and the second frequency
difference may comprise: generating a first electric signal having
a frequency corresponding to the third frequency difference based
on the fourth optical signal; generating a second electric signal
having a frequency corresponding to the fourth frequency difference
based on the fifth optical signal; and detecting a phase difference
between the first and second electric signals.
Also, a method for synchronizing phases of optical signals related
to the present invention uses any of the above methods, wherein:
the first frequency difference is variable and controlled; the
second frequency difference is a reference as a goal of control;
and the method further comprises changing the first frequency
difference based on the phase difference.
Changing the first frequency difference may comprise: generating an
optical comb having a frequency interval corresponding to the phase
difference; and extracting the first and second light waves from
the optical comb, and the method may further comprise extracting
two light waves, from the optical comb, including at least one
light wave which is different from either of the first and second
light waves.
Further, a device for synchronizing phases of optical signals
related to the present invention uses any of the above methods and
comprises: a two light wave generator for generating the first
optical signal; an optical coupler for combining the first and
second optical signals; and an optical separator for separating the
third optical signal into the fourth and fifth optical signals.
The device may further comprise: a first photodetector for
generating a first electric signal having a frequency corresponding
to the third frequency difference based on the fourth optical
signal; and a second photodetector for generating a second electric
signal having a frequency corresponding to the fourth frequency
difference based on the fifth optical signal, and upper limits of
detectable frequency ranges for the first and second photodetectors
may be lower than the first frequency difference.
According to the method for comparing frequency differences between
optical signals, the method for synchronizing phases of optical
signals and the device for synchronizing phases of optical signals
related to the present invention, two optical signals are combined
and separated for corresponding light wave pairs, and then
frequency differences are detected and compared. Accordingly, the
frequency range for which optical-electric conversion is required
is limited so that the number of converters required is
reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an exemplary construction of an optical oscillator
related to a first embodiment of the present invention.
FIG. 2 shows a spectrum of a third optical signal.
FIG. 3 shows an exemplary construction of an optical oscillator
related to a second embodiment of the present invention.
FIG. 4 shows an exemplary construction of an optical oscillator
related to a third embodiment of the present invention.
FIG. 5 shows an example of a conventional construction for
comparing frequency differences.
FIG. 6 shows another example of a conventional construction for
comparing frequency differences.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First, an overview and operational principle of the invention are
explained. The present invention is related to a method for
comparing frequency differences between optical signals, a method
for synchronizing phases of optical signals and a device for
synchronizing phases of optical signals. This invention allows the
extraction of reference signal phase, without using any
high-frequency photomixer, from a high-frequency reference signal
transmitted by two light waves and phase-synchronizing an optical
signal oscillator which generates two light waves to be
controlled.
The basic approach of the present invention is that a phase
difference between a high-frequency optical reference signal and a
signal outputted from an optical oscillator can be measured as
phase differences detected by using two respective low-frequency
photodetectors, without subjecting the high-frequency optical
reference signal to optical-electric conversion directly, by using
an optical frequency shifter or by slightly displacing a wavelength
of a laser source for the two light wave generation means. With
this approach, synchronization with the high-frequency optical
reference signal ranging from a low frequency to a high frequency
can be realized by a small number (e.g. two) of identical
low-frequency photodetectors.
First Embodiment
Hereinafter, a first embodiment of the present invention will be
explained. FIG. 1 shows an exemplary construction of an optical
oscillator 10 related to the first embodiment of the present
invention. The optical oscillator 10 comprises a two light wave
generation means 11, an optical distributor 12, an optical-electric
converter 13, an optical frequency shifter 14, an optical coupler
15, an optical separator 16, photodetectors 17 and 18 and a phase
difference detector 19. These components excepting the
optical-electric converter 13 constitute a phase locked loop. The
optical oscillator 10 compares frequency differences between
optical signals or synchronizes the optical signals by using a
method described below.
Note that, since frequency and phase can be mutually converted
through a differentiation or integration operation, frequency and
phase may herein be treated as equivalents. In the following, the
terms "frequency difference" and "phase difference" are
interchangeable.
The two light wave generation means 11 operates by using a laser
(not shown) for example. The two light wave generation means 11
generates optical signal L1 (first optical signal). The optical
signal L1 includes light wave W1 (first light wave) and light wave
W2 (second light wave). In the example of FIG. 1, the frequency of
the light wave W1 is lower than the frequency of the light wave W2.
The light waves W1 and W2 have frequencies spaced apart by
frequency difference .DELTA.f1 (first frequency difference). The
light waves W1 and W2 (and the light waves described below) are
comprised by respective single frequency components.
Note that, although "frequency component" herein ideally means a
component including a single frequency only, this may substantially
mean a component having a width permitting realization of functions
of the optical oscillator 10 described herein.
Also, for facilitating explanation, light including a single
frequency component only is herein referred to as a "light wave"
whereas light including a plurality of frequency components is
herein referred to as an "optical signal" so that they may be
distinguished. In particular, the term "optical signal" maymean a
signal represented by a frequency difference between two light
waves. However, these terms are not limited to such meanings and a
light wave may, for example, include a plurality of frequency
components.
The frequency difference .DELTA.f1 is variable and controlled. For
example, two light wave generation means 11 can change .DELTA.f1 in
response to a predetermined control signal (this is for example the
undermentioned control signal S2, but another control signal not
shown in the drawings may additionally be used) inputted
externally. Such two light wave generation means 11 can be realized
by using a two light wave generator that uses a Mach-Zehnder type
optical modulator driven by a synthesizer. A specific example of a
construction for the Mach-Zehnder type optical modulator is
disclosed, for example, in "High Extinction Ratio Mach-Zehnder
Modulator Applied to a Highly Stable Optical Signal Generator"
(IEEE Trans. Microwave Theory and Techniques, vol. 55, no. 9, pp.
1964-1972, 2007) by Kiuchi, H., Kawanishi, T., Yamada, M.,
Sakamoto, T., Tsuchiya, M., Amagai, J. and Izutsu, M.
In another example, the two light wave generation means 11 may be
constructed by using a PLL-type device. In the PLL type, an optical
coupler is used to generate the optical signal L1 by combining a
highly stable laser and a variable wavelength laser. The optical
signal L1 is distributed in two paths, one of which being an output
and the other being fed back for controlling the wavelength of the
variable wavelength laser. The feed-back is performed, for example,
by a construction wherein the optical signal L1 is first converted
into a microwave signal by a photomixer, then the microwave signal
and a control microwave signal that varies in response to an
external control signal are combined by a microwave harmonic mixer
or the like, then necessary frequencies are extracted by operating
a filter on the combined microwave signal, and then the extracted
signal is inputted to a generating device for the wavelength
variable laser.
In a further example, the two light wave generation means 11 may be
constructed by using an SSB (Single Side Band) modulator-type
device. In this example, an SSB modulator generates the optical
signal L1 by modulating an input laser (which does not have to be a
highly stable laser) based on a modulation signal (a sine signal
and a cosine signal). The modulation signal (the sine signal and
the cosine signal) may for example be generated by using a 90
degree hybrid circuit and based on a control microwave signal that
varies in response to an external control signal.
In a still further example, the two light wave generation means 11
may be constructed by using an AO (Acousto-Optic element)
modulator-type device. In this example, an AO modulator generates
the optical signal L1 by modulating an input laser (which does not
have to be a highly stable laser) based on a control microwave
signal that varies in response to an external control signal.
The optical distributor 12 distributes the inputted optical signal
L1 into two outputs. One of the distributed outputs is inputted to
the optical-electric converter 13 and the other is inputted to the
optical frequency shifter 14.
The optical-electric converter 13 performs optical-electric
conversion on the optical signal L1 and generates an electric
signal as an output signal S1. Based on the frequency difference
between the light waves W1 and W2 included in the optical signal
L1, it outputs an electric signal having a frequency corresponding
to the frequency difference. In this manner, the optical oscillator
10 related to the first embodiment can output an electric signal
having a desired frequency.
The optical frequency shifter 14 shifts the frequency of the
optical signal L1. That is, it shifts frequencies of the light
waves W1 and W2 by the same amount. In the example of FIG. 1, the
frequency of the optical signal L1 is shifted in a low-frequency
direction so that an optical signal L1' including light waves W1'
and W2' is generated. The frequency difference between the light
waves W1' and W2' is equal to .DELTA.f1.
The amount of this shift (the shifting frequency) is a frequency
greater than the instability in the two light wave generation means
11. In other words, the shifting frequency has a value greater than
the width of the range wherein the frequency difference between the
light waves W1 and W2 varies by an error or the like. Also, the
shifting frequency is less than .DELTA.f1 and preferably much less
than .DELTA.f1. The shifting frequency is, for example, designed so
that its value is included in an operational frequency ranges of
the undermentioned photodetectors 17 and 18.
Here, although the optical signals L1 and L1' include respectively
different frequency components, their respective frequency
differences between their frequency components are both .DELTA.f1
and equal to each other, so they can be regarded as essentially
equivalent optical signals in the optical oscillator 10.
Accordingly, with respect to the present invention, the optical
signals L1 and L1' can both be regarded as the first optical
signal. Also, in a similar manner, the light waves W1 and W1' can
both be regarded as the first light wave and the light waves W2 and
W2' can both be regarded as the second light wave.
The optical coupler 15 combines the optical signal L1' and the
optical signal L2 (second optical signal). The optical signal L2 is
a predetermined optical reference signal inputted externally. The
optical signal L2 includes light wave W3 (third light wave) and
light wave W4 (fourth light wave). In the example of FIG. 1, the
frequency of the light wave W3 is lower than the frequency of the
light wave W4. The light waves W3 and W4 have frequencies spaced
apart by frequency difference .DELTA.f2 (second frequency
difference). .DELTA.f2 is inputted as a goal of control (e.g. as a
frequency reference for an output signal to be generated by the
optical oscillator 10).
The optical coupler 15 generates and outputs an optical signal L3
(third optical signal) by combining the optical signals L1' and L2
in this manner. The optical signal L3 includes the light waves W1',
W2', W3 and W4.
FIG. 2 shows a spectrum of the optical signal L3. As described
above, the frequency difference between the light waves W1' and W2'
is .DELTA.f1 and the frequency difference between the light waves
W3 and W4 is .DELTA.f2. Here, denoting the frequency difference
between the light waves W1' and W3 by third frequency difference
.DELTA.f3 and the frequency difference between the light waves W2'
and W4 by fourth frequency difference .DELTA.f4, it is clear from
FIG. 2 that .DELTA.f1-.DELTA.f2=.DELTA.f3-.DELTA.f4.
The wavelength of the laser as a light source for the two light
wave generation means 11 and the wavelength of the laser as a light
source for the optical reference signal (the optical signal L2)
have respectively different phase instabilities (fluctuations).
However, errors due to the phase instabilities do not affect values
calculated by the above equation because the errors are canceled by
each other. If we explain this referring to FIG. 2, the light waves
W1' and W2' have the same fluctuation so that they fluctuate
together and the light waves W3 and W4 have the same fluctuation so
that they fluctuate together, so their fluctuations can be canceled
by taking the differences as in the above equation. Those skilled
in the art would easily understand such principle based on FIG.
2.
The optical oscillator 10 compares .DELTA.f1 and .DELTA.f2 based on
.DELTA.f3 and .DELTA.f4. Although this comparison may be performed
in any manner, it is performed as described below in the example of
FIG. 1.
The optical separator 16 separates the optical signal L3 into an
optical signal L4 (fourth optical signal) and an optical signal L5
(fifth optical signal). Here, the optical signal L4 includes the
light waves W1' and W3 and the optical signal L5 includes the light
waves W2' and W4. Such an operation can be realized for example by
extracting a frequency component less than a predetermined
frequency threshold and another frequency component greater than
the frequency threshold respectively from the optical signal L3. In
the example of FIG. 2, a frequency at the midpoint of the
frequencies of the light waves W3 and W2' can be chosen as the
frequency threshold.
One of the outputs separated by the optical separator 16 is
inputted to the photodetector 17 and the other is inputted to the
photodetector 18. The photodetector 17 generates an electric signal
E1 (first electric signal) having a frequency corresponding to
.DELTA.f3 based on the optical signal L4. For example, the
photodetector 17 performs optical-electric conversion on the
optical signal L4 and generates the electric signal E1 as an
electric signal having a frequency equal to .DELTA.f3. Similarly,
the photodetector 18 generates an electric signal E2 (second
electric signal) having a frequency corresponding to .DELTA.f4
based on the optical signal L5. For example, the photodetector 18
performs optical-electric conversion on the optical signal L5 and
generates the electric signal E2 as an electric signal having a
frequency equal to .DELTA.f4.
Here, the photodetectors 17 and 18 only have to detect the
frequency differences .DELTA.f3 and .DELTA.f4 in the optical
signals L4 and L5 respectively, regardless of the frequency
difference .DELTA.f1 in the original optical signal L1.
Accordingly, operational frequency ranges of the photodetectors 17
and 18 can be designed conforming to a variable range of .DELTA.f3
or .DELTA.f4 (e.g. from about several MHz to about several hundred
MHz) regardless of a variable range of .DELTA.f1 (which can span a
wide range from a low frequency to a frequency of more than 100
GHz), so it can be covered by a smaller number of photodetectors
(for example a single detector). In other words, the number of
photodetectors required can be reduced by designing upper limits of
detectable frequency ranges for the photodetectors 17 and 18 to be
lower than (preferably much lower than) .DELTA.f1.
The phase difference detector 19 compares .DELTA.f3 and .DELTA.f4
based on the electric signals E1 and E2. Although operation of the
comparison may be performed in any manner, it is performed in the
present embodiment by detecting a phase difference between the
electric signals E1 and E2.
Thus, in the first embodiment, the photodetectors 17 and 18 and the
phase difference detector 19 function as a comparison device
(comparison means) for comparing .DELTA.f1 and .DELTA.f2 based on
.DELTA.f3 and .DELTA.f4.
Although the result of comparison between .DELTA.f3 and .DELTA.f4
may be outputted or utilized in any manner, the result is outputted
as a control signal S2 in the example of FIG. 1 and used for
controlling an operation of the two light wave generation means 11.
For example, the two light wave generation means 11 changes
.DELTA.f1 in the optical signal L1 based on the phase difference
between the electric signals E1 and E2 represented by the control
signal S2. For example, the change is made so that .DELTA.f1 equals
.DELTA.f2 or .DELTA.f1 gets closer to .DELTA.f2. According to such
control, the optical oscillator 10 can synchronize the phase of the
optical signal L1 with the phase of the optical signal L2.
Note that the shifting operation by the optical frequency shifter
14 causes an identical effect on the light waves W1 and W2, so the
effect is removed in the detection operation at the phase
difference detector 19 and does not affect (at least directly) the
control signal S2.
Thus, as described above, in accordance with the optical oscillator
10 related to the first embodiment of the present invention, the
number of photodetectors required can be reduced because the
operational frequency ranges of the photodetectors 17 and 18 can be
designed conforming to the variable range of .DELTA.f3 or .DELTA.f4
regardless of the variable range of .DELTA.f1.
Also, in accordance with the optical oscillator 10, the features
described below are realized. The optical oscillator 10 obtains a
difference between the phase of the high-frequency optical
reference signal (optical signal L2) and the phase of the signal
outputted by the optical oscillator 10 (the optical signal L1)
without performing optical-electric conversion directly on the
optical signal L2. In order to realize this, the optical oscillator
10 uses the optical frequency shifter 14 to slightly displace the
wavelength of the laser which is the light source for the two light
wave generation means 11 so that the difference between the
frequencies detected respectively by the two low-frequency
photodetectors is measured.
The optical oscillator 10 inputs the high-frequency optical
reference signal directly as an optical signal rather than
converting this into an electric signal at the original high
frequency.
The optical oscillator 10 can support high-frequency optical
reference signals from a low frequency to a high frequency (e.g.
from the microwave region to the terahertz region) with two
identical low-frequency photodetectors.
The optical oscillator 10 can perform phase comparison at a low
frequency, so no high-frequency component such as a high-frequency
photomixer or a high-frequency mixer is required within the phase
locked loop.
Second Embodiment
In the first embodiment, the output of the phase difference
detector 19 is inputted to the two light wave generation means 11
directly. In the second embodiment, a component for manipulating a
signal is added therebetween. Differences from the first embodiment
are described below.
FIG. 3 shows an exemplary construction of an optical oscillator 110
related to the second embodiment. The optical oscillator 110
comprises a phase difference detector 122 in addition to the
construction of optical oscillator 10 related to the first
embodiment. Inputted to the phase difference detector 122 are a
phase difference signal S102 as an output of the phase difference
detector 19 and an offset frequency signal S103 of a frequency
corresponding to a predetermined offset. The offset frequency
signal S103 is, for example, an electric signal with a microwave
frequency. The phase difference detector 122 detects and outputs
the phase difference between these two signals. The output is, for
example, inputted to the two light wave generation means 11 as a
control signal S104. The two light wave generation means 11 changes
.DELTA.f1 in response to the control signal S104.
In accordance with such a construction, control of the .DELTA.f1
can be performed more flexibly. For example, .DELTA.f1 can be
changed dynamically whereas .DELTA.f2 remains fixed by controlling
the frequency of the offset frequency signal S103.
Also, the number of photodetectors required can be reduced in a
manner similar to the optical oscillator 10 related to the first
embodiment because the operational frequency ranges of the
photodetectors 17 and 18 can be designed conforming to the variable
range of .DELTA.f3 or .DELTA.f4 regardless of the variable range of
.DELTA.f1.
Third Embodiment
In the first and second embodiments, the optical signal inputted to
the optical-electric converter 13 and the optical signal inputted
to the optical frequency shifter 14 are identical optical signals
L1. In the third embodiment, an optical comb is used so that
different pairs of light waves are extracted and inputted to the
optical-electric converter 13 and the optical frequency shifter
respectively. Differences from the second embodiment are explained
below.
FIG. 4 shows an exemplary construction of an optical oscillator 210
related to the third embodiment. The optical oscillator 210
comprises an optical comb generation means 202, an optical
distributor 212, a first two light wave selection means 211a and a
second two light wave selection means 211b instead of the two light
wave generation means 11 and the optical distributor 12 in the
optical oscillator 110 related to the second embodiment.
The optical comb generation means 202 generates an optical comb L6
based on the control signal S104 inputted from the phase difference
detector 122. The optical comb L6 includes at least three light
waves. The light waves are spaced apart by a frequency difference
f.sub.comb to the frequency of the control signal S104. Here, the
frequency of the control signal S104 is determined based on the
phase difference between the electric signals E1 and E2, so it can
be said that the optical comb L6 has a frequency interval
corresponding to the phase difference.
The optical distributor 212 distributes the inputted optical comb
L6 into two outputs. One of the distributed outputs is inputted to
the first two light wave selection means 211a and the other is
inputted to the second two light wave selection means 211b.
The first two light wave selection means 211a selects and extracts
two adjacent light waves (corresponding to the light waves W1 and
W2 in the first embodiment) from the optical comb L6 and outputs an
optical signal including the two light waves as the optical signal
L1. Thus, the frequency difference in the optical signal L1
(.DELTA.f1 in FIG. 1) is equal to f.sub.comb. The optical signal L1
is inputted to the optical frequency shifter 14 in a manner similar
to the first and second embodiments.
The second two light wave selection means 211b selects and extracts
two predetermined light waves from the optical comb L6 and outputs
an optical signal L7 including the two light waves. The frequency
difference in the optical signal L7 is thus selectable at will, in
response to the operation of the second two light wave selection
means 211b, from among f.sub.comb.times.n where n is an integer
equal to or greater than 2. In the example of FIG. 4, n=3. n may be
dynamically changeable.
By setting n to be 2 or greater, each two light wave selection
means would extract respective different pairs of light waves. In
other words, at least one of the two light waves extracted by the
second two light wave selection means 211b would be different from
either of the two light waves extracted by the first two light wave
selection means 211a (which correspond to the light waves W1 and W2
in the first embodiment).
The optical signal L7 is inputted to the optical-electric converter
13. The optical-electric converter 13 performs optical-electric
conversion on the optical signal L7 in a manner similar to the
first and second embodiments to generate an electric signal as the
output signal S1.
In accordance with such a construction, the optical oscillator 210
comprising a frequency multiplication function can be realized.
That is, the frequency difference in the optical signal L7 is n
times the frequency difference .DELTA.f2 in the optical signal
L2.
Also, the number of photodetectors required can be reduced in a
manner similar to the optical oscillator 10 related to the first
embodiment because the operational frequency ranges of the
photodetectors 17 and 18 can be designed conforming to the variable
range of .DELTA.f3 or .DELTA.f4 regardless of the variable range of
.DELTA.f1.
Although n is equal to or greater than 2 in the above third
embodiment, the case of n=1 may be allowed. Also, although the
first two light wave selection means 211a extracts two adjacent
light waves from the optical comb L6 in the third embodiment, it
may be constructed so that it can extract two non-adjacent light
waves. For example, it may be constructed so that the frequency
difference can be selected at will from among f.sub.comb.times.m
where m is an integer equal to or greater than 1. In this case, it
may be set so that m>n or m=n. In particular, in the case
wherein m=n, an optical oscillator substantially equivalent to the
second embodiment is realized.
In the first to third embodiments above, the frequency shift is
performed by the optical frequency shifter 14 on the optical signal
L1. In a variant, the frequency shift may be performed in a
different manner if it is performed before the optical signal L3 is
generated. For example, this may be performed by slightly
displacing the wavelength of the laser which is the light source
for the two light wave generation means 11 or the optical comb
generation means 202 (in which case the optical frequency shifter
14 is not required) or may be performed by an optical frequency
shifter on the optical signal L2. Note that, in a construction
wherein the frequency of the optical signal L2 (light waves W3 and
W4) is shifted, both the optical signal L2 and the shifted version
thereof can be regarded as the second optical signal.
The present invention can for example be applied in the fields of
high-frequency astronomy and ultra-high-speed communication. Also,
the present invention can be applied in fields utilizing waves from
microwaves to terahertz waves.
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